Kids love playing with water, so it's a simple, natural choice for science experiments. You can ask the children to put different objects in the water to see which ones float and which ones sink. Ask them first to guess what will happen, so they get a basic idea of the components of a science experiment. Or, experiment with senses and water temperature. Have each child put one hand in a pot of cold water and the other in a pot of very warm water. Then have them put both hands in a pot of lukewarm water. One hand will feel cold and the other hot.
Freeze water in small ice cube trays and a larger container, and ask the children to guess which one will freeze faster. Bring them outside the freezer once both are frozen, then ask which will melt faster. Set out two pieces of ice of the same size, and keep one inside and one outside. Predict which will melt faster based on the temperature inside vs. the temperature outside. Or, set an ice cube on a plate and ask if each child can pick it up with a piece of string. Then sprinkle some salt on the ice cube and repeat. Explain how the salt slightly melts the ice and refreezes, "gluing" the string to the ice.
Basic chemistry involves teaching how items interact with each other when combined. Drop a few raisins into a glass of clear club soda, and show how the raisins "ride" the bubbles up and down. Or, in a clear bowl, dye water blue with food coloring. Add oil and show how the oil and water do not mix. For a fun science demonstration, put water into a bowl and sprinkle it with black pepper. Add a drop of dish soap and have the children watch how the pepper is repelled to the sides of the glass.
Have the children soak beans in water overnight. The next day, plant them in clear plastic cups in soil. Water them, put them by a window, and watch the plants grow very quickly. Discuss what the plants need, and experiment further by using different amounts of water in different plants and exposing them to different amounts of light. Or, get a white cut flower, such as a carnation, and place it in a clear vase of water. Add food coloring to the water, and eventually the flower will turn the color of the food coloring. This illustrates how plants get water.